Color imaging with a digital camera (in both still and video mode) as known today is performed using a pixelated image sensor that has a CFA, most often a Bayer pattern of R (red), G (green) and B (blue) filters. For simplicity, such a camera will be referred to henceforth as “RGB camera”. The CFA pixels are arranged to match image sensor pixels. With the decreasing size of modern image sensor pixels (which approaches the wavelength of light), the signal level for a given photon flux per pixel decreases and the capture of each photon becomes crucial. Color filters prevent most (in some cases ca. 70%) of the photons from reaching the image sensor and therefore affect significantly such parameters as sensitivity and signal-to-noise ratio (SNR). Moreover, the fabrication of CFAs, which requires a number of masking and deposition stages, is costly.
A color image is essentially a weighted combination of RGB “band” images. Thus, a color image can be “constructed” (or “reconstructed”) to match an original imaged object if its RGB components are known. Each R, G and B band is itself a weighted combination of many separate spectral or hyperspectral (“HS”) images at distinct wavelengths (or bands) within the R, G or B band. As used herein, the term “hyperspectral” refers exemplarily to more than ca. 10 wavelengths. Consequently, a RGB color image may be reconstructed from spectral or hyperspectral image data, for example as described in D. H. Foster et al., “Frequency of metamerism in natural scenes”, Journal of the Optical Society of America A, 23, 2359-2372 (2006). However, such reconstruction is currently severely limited by the time and processing resources needed to acquire the HS data and by the time and processing resources needed to reconstruct the color image from the HS data.
PCT patent application PCT/IB2014/062270, filed 16 Jun., 2014 by the present inventors and titled “Apparatus and method for snapshot spectral imaging” teaches snapshot (single shot) HS imaging using a monochromatic digital camera that has a minimal hardware addition in the form of a restricted isometry property (RIP) diffuser element. The digital camera is adapted to provide a large number of spectral images in a snapshot. The spectral images are reconstructed from a single diffused-dispersed (DD) image, which is a single image obtained at the image sensor through the camera and the RIP diffuser. A hardware or software randomizer may be added to the camera with the RIP diffuser to provide a single diffused-dispersed and randomized (DDR) image at the image sensor. The reconstruction of spectral images from a single DD or DDR image described in PCT/IB2014/062270 is performed using compressed sensing (CS) algorithms. More specifically, PCT/IB2014/062270 teaches two dimensional (2D) CS-based spatial-spectral cube reconstruction (SCR) or “2D CS-SCR”.
There would be clearly a tremendous advantage in terms of both camera performance and image sensor fabrication costs if color images in both still and video mode could be obtained with a monochromatic digital camera having a “clear” image sensor that does not have color filters. Moreover, there is a need for and it would be advantageous to have monochromatic digital cameras and associated methods that can provide such images in real time and with restricted processing resources.